The head of Britain's armed forces on Sunday said the country was "letting down" allies by not participating in air strikes against Islamic State group jihadists in Syria.

Britain is part of a coalition hitting IS targets in Iraq, but its parliament has yet to be convinced of the need to join air strikes in neighbouring Syria and an influential committee of MPs recently advised against action.

"To an extent yes, we are letting our allies down by not being a full player," General Nicholas Houghton, the chief of the defence staff, told Sky News on Sunday.

"The source of their power, their command and control, their logistics, their organisation, the place from which they issue orders to international terrorists is from within Syria.

"In the most simplistic way it's like being asked to win a football match but not being able to go into the opponents' half," he added.

However, the defence chief stressed that Britain could make only "make a contribution to the international" effort.

"In the end the military dimension is done by regional platers, by Muslim countries," he later told the BBC.

"I don't think we should play the decisive military role because it runs the risk of adding fuel to the radicalisation of IS as an abhorrent cult."

Foreign Secretary Philip Hammond later denied that the government had given up on a parliamentary vote.

"We are in exactly the same place that we've been for months," he told the BBC.

"When we think its right to do so, when we think we have a consensus, we will go to the House of Commons."

Hammond said the government was still in an "exploration" period with the opposition Labour party, which recently elected leftist Jeremy Corbyn as its leader, and was yet to establish if it had the cross-party support required to win a vote to extend the air strikes.

He admitted that Britain's decision "wasn't going to tip the balance" in Syria and called for a political solution to bring about a ceasefire in the four-year civil war.

- 'Illogical' -

The government has argued that it is illogical to conduct air strikes in Iraq and not neighbouring Syria, saying the two countries are "a single theatre of conflict".

But the Commons Foreign Affairs Committee -- a cross-party MPs' body which scrutinises Britain's foreign policy -- said in a new report that Prime Minister David Cameron's focus on joining air strikes was "incoherent" and "a distraction".

However, the crash of a Russian passenger jet in Egypt, which Britain believes was probably the result of a bomb, has once again thrust the issue centre stage.

Britain has suspended scheduled flights out of Sharm el-Sheikh as a result, and has sent over aircraft to fly home Britons remaining in the resort.

That decision was initially criticised by Russia, which is already hitting targets in Syria, but Houghton said Sunday that the common enemy of IS presented "an opportunity for an element of political convergence between America, ourselves, the West and (Russian President Vladimir) Putin."

Hammond on Sunday warned that air passengers worldwide faced more security checks, "additional costs and additional delays," if it were found that an IS bomb was responsible.

In Iraq, Britain is currently part of a coalition of more than 60 countries and has eight Tornado jets flying missions plus an unconfirmed number of Reaper drones.

This was approved by parliament in September last year, but the government was defeated on a vote to strike Syria in 2013.